Devices for x-ray imaging feature automatic exposure control. This insures that an adequate signal strength is always present at the input of an x-ray detector. This is achieved by setting various x-ray parameters, above all through an adjustment of the tube voltage, of the tube current, of the tube pre-filter and of the pulse width of the x-ray radiation.
Thus, if the absorption behavior of the object to be examined or irradiated changes, through a change in the fluoroscopy angle for example, the aforesaid parameters are adjusted automatically, whereby a constant signal strength is guaranteed at the input of the x-ray detector.
Each change in the x-ray parameters is also associated with disadvantages however, which have a negative influence on an x-ray imaging, such as for example:                a lower image contrast for an increased signal caused by a higher tube voltage,        a greater x-ray focus and thus a lower spatial resolution through an increased signal caused by a higher tube current,        a higher skin dose through an increased signal caused by a lower x-ray pre-filtering and also        a lower time resolution through an increased signal caused by a longer pulse width of the x-ray radiation.        
The adjustment of the x-ray parameters is generally governed by a fixed scheme, which is applied as a basis for regulating the x-ray parameters. Specific requirements for the control behavior, for example a short pulse width for a cardiological application, are merely taken into account by corresponding limit values, such as for example a pulse width of less than 10 ms. This limit value will not be exceeded when the x-ray parameters are changed and the application-specific requirement for the x-ray images is insured, such as for example a high temporal resolution for a cardiological application.
However only a generally valid application-specific adjustment of the pulse width is possible through such fixed limit values, an actual patient-specific situation cannot be taken into account. It is precisely during a cardiological application, in which a high time resolution is briefly not required, that this leads to sub-optimal conditions. For example an increased signal also occurs then on the basis of the usual x-ray parameters, although a longer pulse width would have fully provided an acceptable image quality.